Components of a rotary piston machine

ABSTRACT

The friction surface of a cast iron side wall member for a rotary piston engine is hardened by iontriding in an electrical glow discharge in a gas atmosphere containing nitrogen, after coating the areas not to be hardened, to inhibit glow discharge at those areas.

BACKGROUND OF THE INVENTION

The present invention relates to an iron or steel component, subject tofriction, for a rotary piston machine, particularly to its double-walledside member, central portion or trochoid housing made of cast iron. Thepresent invention furthermore relates to a process of producing suchcomponents subject to friction.

In the operation of such rotary piston machines the maximum achievableoperational life is essentially determined by the components subject tofriction. These components possess large flat surface areas over whichthe rotating piston wipes with its sealing strips so that acomparatively large frictional stress occurs. Since the rotary pistonedges or sealing strips wiping over these surface areas must ensure agas-tight seal, only small wear of these areas is admissible.Accordingly, it has for some time been attempted to make thesecomponents, or at least their surface areas subject to friction, aswear-resistant as possible. However, this problem is difficult to solvebecause it involves complex components, particularly because thetwo-walled side or central portions are provided with a large number ofbores and connections. If at all, such double-walled cast-ironcomponents with major wiped surfaces subject to friction may, however,be treated with the previously usual processes for surface hardeningonly with great difficulty.

Hardening of the wiped areas of such components has so far been effectedwith varying results in well-known metal spraying processes which,however, constitute a most expensive treatment which is moreoverdangerous owing to possible adverse effects on health. Apart from thefact that the raw materials employed, such as molybdenum, can often onlywith difficulty be obtained in the quantities required for the treatmentof a large number of components, the wiped surfaces provided with such acoat must subsequently be reground, which results in a further costincrease. Reworking such coated areas following a certain period ofoperation of the rotary piston machines has so far not been possible.

Again, the so-called induction hardening of such wiped areas ispossible, but in view of the great stresses arising in the operation ofsuch rotary piston machines such hardened wiped surfaces are notsufficiently wear-resistant in order to ensure satisfactory operationallife. The deformations occurring in such a treatment are in addition sopronounced that they require regrinding and can hardly be eliminatednonetheless.

Experiments made with salt-bath nitriding and also with thermal gasnitriding produced no acceptable result because the hardening processwas there not limited to the wiped surfaces proper but resulted insubjecting the components in question to nitriding as a whole. In viewof the complex double-walled castings with their irregular bores,experience has shown that deformation occurs so that the desired highprecision of the wiped areas is no longer ensured. In addition, theabsorption of certain quantities of nitrogen in the base materialresults in a change in size, which cases substantial difficulties inrespect of preserving the necessary dimensional accuracy. Moreover,residues of the nitriding bath, which can virtually not be removed, areformed particularly in salt-bath nitriding complex hollow components.

Despite the failure of the common nitriding processes, applicant hasundertaken to achieve the desired properties using the ionitridingprocess. In so doing, it has surprisingly found that it is possible inso-called ionitriding by means of an electrical glow discharge in a gasatmosphere containing nitrogen, to produce, with adequate control of theionitriding process, such components which have outstandingly goodrunning properties without necessitating reworking of the componentsthat have previously been processed to their final dimensions.

The present invention relates to such an iron or steel component, whichis subject to friction, for a rotary piston machine, particularly forits double-walled side unit, central unit or trochoid housing made ofcast iron, which is characterized in that the surfaces limited to thewiped areas are hardened by ionitriding with an iron nitride zonereduced to less than 10 microns thickness and with a surface roughnessof a maximum of R₂ = 0.8 micron, with a corrosion resistance relative tocombustion residues of fuels increased by at least 300 percent, with athermal resistance up to 500°C over long periods or up to 800°C overshort periods, and with a ductility reduced by a maximum of 10 percentas compared with the basic material.

The invention furthermore relates to a method of producing the componentsubject to friction according to this invention, ionitriding beingperformed by means of an electrical glow discharge in a gas atmospherecontaining nitrogen at an elevated temperature in a treatment chamber,characterized in that the said component is provided with a coatinginhibiting the electrical glow discharge on the surfaces outside thewiped surfaces proper, then ionitriding the surfaces of the wiped areasafter inserting the component in the treatment chamber for at least fivehours at a gas pressure of 0.1 - 5 mmHg and an operating voltage above500 Volts, the said operating voltage and the gas pressure being socontrolled that the iron nitride zone is partly removed by spatteringmetal particles from the surface and the thickness of the zone thus heldbelow 10 microns.

The invention is explained in greater detail in FIGS. 1 through 4 of theenclosed drawing in which

FIG. 1 shows a double-walled side driving disk of a rotating pistonmachine,

FIG. 2 a double-walled trochoid housing of a rotary piston machine,

FIG. 3 a double-walled central unit of a rotary piston machine, shownpartly in section,

FIG. 4 a diagram of various wear resistances of nitrided wiped surfaces.

The components diagrammatically shown in FIGS. 1, 2 and 3 for single,double and multi-chamber rotary piston machines are double-walled hollowbodies made of iron or steel, preferably cast iron. With the componentsshown in FIGS. 1, 2 and 3 the wiped areas of the visible portions aredesignated at 5 and those of the invisible portions indicated by thedot-dash line 6. In operation, the rotating piston wipes these wipedareas with its sealing edges and, respectively, sealing strips so thatthe entire wiped area is subject to friction. The numerals 7, 8, 9 and10 indicate some of the numerous perforations and connections which arerequired to supply and remove cooling water, fuel mixture, exhaust gasesand the like. With such components of a rotary piston machine as showndiagrammatically in FIGS. 1, 2 and 3, the wiped areas must naturally becompletely smooth and, with the double sides of the central portion asper FIG. 3 they must be perfectly plane parallel on the front and,respectively, rear flat sides; any deformation of the complex hollowbodies during the hardening process would make the wiped areas and,respectively, the entire component useless.

With complex components of the type shown in FIGS. 1, 2 and 3, hardeningof the wiped areas by means of bath nitriding is virtually impossible ifonly owing to the unavoidable residues, which can hardly be removed fromthe inside of the hollow body; at the same time the unsymmetricallyarranged openings would render it impossible to avoid pronounceddeformation during the nitriding treatment. Again, in thermal gasnitriding the openings 7, 8, 9, 10 would cause heavy deformation of theentire component. In addition, the two said niriding methods wouldresult in that the distance between the plane parallel wiped areas wouldbe increased owing to the absorption of nitrogen by the basic materialso that additional processing of the said wiped surface would berequired. In contradistinction, ionitriding of components of the typeshown in FIGS. 1, 2 and 3 provides the possibility of equipping allsurfaces located outside the wiped areas proper with suitable coatingswhich prevent the occurrence of an electrical glow discharge at thosepoints and thus the penetration of nitrogen into these parts of thesurface. For a coating, mechanical coatings or also a coating made by anapplicable compound may, by way of example, be selected, which serve thepurpose. The said coatings may also be limited to the direct vicinity ofthe openings since it is only they that would cause deformations of thecomponent during nitriding; this applies particularly to insert diskswhich may be provided in the areas of the sides subject to wear in thebodies according to FIGS. 1 and 3.

After coating the areas of the surface not requiring ionitriding, thesaid components of a rotary piston machine or parts thereof are placed,individually or together with other associated components, in thetreatment chamber usual for ionitriding and arranged in such a mannerthat an electrical glow discharge may freely form on the wiped surfacesto be ionitrided. The component should in the usual manner have anegative potential at least occasionally, as described in greater detailin applicant's patent specifications so that lengthy descriptions aresuperfluous.

In ionitriding the wiped surfaces of components, which are subject towear, of rotary piston machines, however, it is of great importance inorder to meet the great demands made of the resistance to wear of thewiped areas that the treatment is effected for at least 5 hours at a gaspressure in the range of 0.1 and 5 mmHg and an operating voltage inexcess of 500 Volts, preferably between 600 and 1,000 Volts. Theoperating voltage and the gas pressure must be so regulated that thespattering of metal particles from the surface of the wiped areas isobtained and that the iron nitride zone created by the ionitridingprocess is partly removed. Despite the comparatively long period oftreatment the iron nitride zone should be kept at below 10 microns bythe simultaneous spattering from the wiped areas. So if the componentsas indicated in FIGS. 1, 2 and 3 are ionitrided in the manner describedabove, the surface hardened by ionitriding is substantially limited tothe wiped areas which are provided with an iron nitride zone of lessthan 10 microns and possess a surface roughness of a maximum of R_(a) =0.8 micron. The Vickers hardness HV 0.2 of the ionitrided surface areasexceeds 450 kp/mm², and the wiped surfaces so ionitrided possessincreased resistance to corrosion against the combustion residues of theusual fuels. The thermal resistance of the wiped areas and,respectively, the components amounts to about 500°C over long periods,but to 800°C and above over short periods. Against the basic materialthe wiped areas possess, despite the great hardness, a ductility whichis reduced by not more than 10 percent. The above-mentioned favorableproperties of the wiped areas hardened by ionitriding permit ofexpecting, in comparison with other surface hardening treatments,greater resistance to wear than those treated in other surface hardeningprocesses. In order to prove the increase in wear resistance, a largenumber of wear tests were performed with Amsel wear testers. FIG. 4shows the results in the form of a diagram.

It was found that, with ionitrided wiped surfaces according to thepresent invention, not only the mechanical wiping wear is smaller butalso such wear as is caused by so-called tribochemical applications inthe wear process. Connected therewith is the fact that wiped areas sotreated also possess a substantially higher resistance to corrosion bycombustion residues of fuels. As previously stated, the wiped surfacesof the components ionitrided according to this invention of rotarypiston machines also possess greater thermal resistance than surfaceshardened by other methods. By way of example, the thermal resistanceobtained in induction hardening is always below 400°C and, in casehardening the thermal resistance hardly exceeds 150°C. On the otherhand, the wiped surfaces treated in accordance with this inventionreveal a thermal resistance of 400°C over extended periods and of 800°Cor higher over short periods.

Life tests were finally performed on rotary piston machines of the oneand two-chamber design with the components according to this invention.It has been found that the components according to this inventionprovide an increase in the operational life of such machines and,respectively, engines in the order of two or three times the originalvalue. It has further been found that used components of this type canbe reconditioned, which constitutes a considerable advantage over thetreatment of the surfaces by spraying them with molybdenum.

What is claimed is: .[.1. A trochoid housing for a rotary pistonmachine, made of cast iron and provided with ionirided hardened areaslimited to the wiped surfaces; charcterized in that the hardened areasthroughout the wiped areas are provided with an iron nitride zonereduced to less than 10 microns thickness, with a surface roughness notexceeding R_(a) = 0.8 micron, with a corrosion resistance relative tofuel combustion residues increased by at least 300 percent, with athermal resistance up to 500°C over long periods and, respectively, upto 800°C over short periods and with a ductility reduced by not morethan 10 percent as compared to the basic material..]. .[.2. A method ofproducing a cast iron trochoid housing for a rotary piston machine inwhich ionitriding is performed by means of an electrical glow dischargein a gas atmosphere containing nitrogen at elevated temperature in atreatment chamber, the said housing being provided with means inhibitingthe electrical glow discharge on the surfaces outside the wiped areasproper prior to ionitriding, characterized in that the said areas areionitrided after insertion in the treatment chamber during at least 5hours at a gas pressure in the range of 0.1 - 5 mmHg and an operatingvoltage of more than 500 volts, the operating voltage and the gaspressure being so controlled that the sputtering of metal particles fromthe surface of the wiped areas partly reduces the iron nitride zone andkeeps it below 10 microns..]. .Iadd.
 3. Trochoid housings, side andcentral units for a rotary piston machine, made of a material selectedfrom the group consisting of iron and steel, particularly cast iron, andprovided with ionitrided hardened areas limited to the wiped surfaces;characterized in that the hardened areas throughout the wiped areas areprovided with an iron nitride zone reduced to less than 10 micronsthickness, with a surface roughness not exceeding R_(a) = 0.8 micron,with a corrosion resistance relative to fuel combustion residues by atleast 300 per cent, with a thermal resistance up to 500°C over longperiods and, respectively, up to 800°C over short periods and with aductility reduced by not more than 100 per cent as compared to the basicmaterial. .Iaddend..Iadd.
 4. A method of producing trochoid housings,side and central units for a rotary piston machine made of a materialselected from the group consisting of iron and steel in whichionitriding is performed by means of an electrical glow discharge in agas atmosphere containing nitrogen at elevated temperature in atreatment chamber, said trochoid housings, side and central units beingprovided with means inhibiting the electrical glow discharge on thesurfaces outside the wiped areas proper prior to ionitriding,characterized in that the said areas are ionitrided after insertion inthe treatment chamber during at least 5 hours at a gas pressure in therange of 0.1 - 5 mmHg and an operating voltage of more than 500 volts,the operating voltage and the gas pressure being so controlled that thesputtering of metal particles from the surface of the wiped areas partlyreduces the iron nitride zone and keeps it below 10 microns. .Iaddend.